RAH Individuals that left the group early (Departure time ≤ 100 sec) were victimized significantly more times per minute than indivi- duals that stayed the longest (Departure time ≥ 200 sec) (p=0.05, df=17). This supported the Regulatory Aggression Hypothesis. EXPLORING TEMPORAL DYNAMICS AND REGULATORY BEHAVIORS OF FORAGING GROUPS OF JUVENILE COHO SALMON Christopher J. Naus, Matthew J. Jacobson, and Dr. David Lonzarich, University of Wisconsin–Eau Claire Acknowledgments: UWEC ORSP for project funding, Dr. Dan Janik for loaning his behavior recording equipment, Don Schleicher for video editing, and Bill Hintz for previous research and insight BACKGROUND Many animals forage in groups, groups offer: Advantages – Decreased risk of predation – Increased feeding rate Disadvantages – Intraspecific competition – Aggression As group size increases, disadvantages begin to decrease individual fitness. However, there are still fitness benefits to living in large groups for lone foragers. Group Size Paradox (Figure 1) Optimal Group Size – Group size at which individual fitness is highest. Equilibrium Group Size – Group size at which fitness for a group member and an individual forager are equal. Paradox: Group size should increase until equilibrium group size is reached. Previous research has shown that group sizes are maintained at an optimal level. (Beauchamp G, and Fernández-Juricic E The group-size paradox : effects of learning and patch departure rules. Behav. Ecol 16:2: ) Optimum Group Size FITNESS GROUP SIZE Equilibrium Group Size PAH Individuals that left early had significantly shorter durations bet- ween foraging events than indi- viduals that stayed the longest (p<0.001, df=17). This is opposite of what the patch assessment hypothesis predicts. We propose two alternative explanations to further examine the patch assessment hypothesis. METHODS Study conducted in the Onion River, Bayfield County, WI. from May-August 2008 Field Methods – Bank shore counts of groups of juvenile coho salmon (Figure 2) Counted 126 foraging groups Group counts made every 30 seconds for 15 minutes Group sizes were distributed equally into categories: 1-3, 4-6, 7-9, , 13-15, 16-18, 19-21, and 22+ – Underwater Video recording Recorded 69 groups for 15 minutes Video Analysis – Four videos from each category were analyzed for as many visible fish as possible in a random five minute video segment. Foraging Aggression ( Perpetrator and Victim) Departure time CONCLUSIONS Foraging groups of juvenile coho salmon are maintained around a mean and highly regulated near an optimal size. Aggression appeared to be an important mechanism of group regulation. Our research suggests tests of the patch assessment hypothesis require more than measuring foraging gains. It is important to consider energy costs and alternative feeding strategies. As group size increases predation risk decreases, fish forage more and occupy more productive space. Declines in per capita consumption as food becomes scarce. ALTERNATE EXPLANATIONS Transient Fish Some fish move from group to group feeding opportunistically until aggression and energy expenditure become high. Energy Expenditure Preliminary evidence showed that fish that left early (transients) moved around more to acquire food. This suggests a lower net energy gain (p<0.001, r 2 =0.40). TEMPORAL DYNAMIC RESULTS Figure 1: Graph illustrating fitness vs. group size relationship and optimal and equilibrium group sizes. Figure 2: Subset of foraging group of juvenile coho salmon (Onchorhynchus kisutch). REGULATORY BEHAVIOR RESULTS Relationship between standard error corrected for group size plotted against group size. Groups near the optimal sizes were highly regulated. STUDY OBJECTIVES Fish forage in groups that vary in size Groups do not randomly fluctuate Group structure is temporally dynamic, but maintained around a mean Optimal group sizes are most stable New members are repelled by resident group members Why? Regulatory Aggression Hypothesis (RAH) Patch Assessment Hypothesis (PAH) Individuals use knowledge of past food patches